Consider a typical public library. Each book has its own place in a particular shelf. Readers may take several books (from different shelves) and browse through them till they find the right book. While some readers manage to return the unwanted books to the correct place, many will either not replace the book at all, or will put it back in the wrong place. The latter is hard to detect; to fix this problem, libraries hire people to “read shelves” periodically to find incorrectly filed books. Similar situation exists in many retail stores where the customers can try several items before deciding which one to buy. In many cases, the customer never returns the tested item to its correct shelf and again, store personnel must spend time putting items back in their correct place. Some kind of automated tracking mechanism would be very useful. The problem of asset tracking is not restricted to libraries or retail stores. Many companies are realizing the importance of increasing the visibility within their supply chain. Asset tracking—knowing what you have and where it is located—is essential for the smooth operation of large manufacturing companies. It also helps big retailers isolate bottlenecks in their supply chain, reduce overstocking or locate spoiled cargo. Similarly, government and military organizations are interested in cheaper (and more efficient) ways to track their assets and equipment.
Automatic location sensing is a key in enabling such tracking applications. One of the best-known location-based systems is GPS, which relies on satellites to track location. However, due to the dependence on low-power satellite signals, GPS is difficult or impossible to use inside buildings to determine location. So, in order to achieve location tracking inside buildings, researchers and industry have proposed several systems that differ with respect to technology used, accuracy, coverage, frequency of updates and the cost of installation and maintenance. Triangulation, scene analysis, and proximity are some of the principal techniques for automatic location-sensing. Many of the current location sensing systems are radio based (Wi-Fi, Bluetooth, ZigBee, UWB). By using base station visibility and signal strength or time of flight, it is possible to locate Wi-Fi devices with an accuracy of several meters. In many situations, however, it is prohibitively expensive to continuously track an item. Continuous tracking may be used in scenarios where the items in question have high value or are of great importance (for example military equipment, jewelry boxes, etc.), thus justifying the cost. However, for many applications (e.g. tracking inventory) a periodic (say nightly) recording of location is sufficient.
In recent years, RFID technology has attracted considerable attention. RFID is emerging as an important technology that is reshaping the functioning of supply chain management. RFID not only replaces the old barcode technology but also provides a greater degree of flexibility in terms of range and access mechanisms. For example, an RFID scanner can read the encoded information even if the tag is concealed for either aesthetic or security reasons. Various companies and governmental agencies are proposing to use RFID for identifying large lots of goods at the pallet and carton level. Usually passive tags (that is, those without their own power source) are preferred for tagging goods as they are much cheaper, long lived, lightweight and have a smaller foot print. However since passive tags work without a battery, they also have a very small detection range and hence are not normally used in location sensing system that are purely RFID-based. Active tags, with their own batteries, have a much greater detection range and might be used as part of a positioning system, but these tags are currently too expensive for wide-spread deployment. What is lacking is an efficient and economical means of detecting the location of the passive tags. Our invention addresses this need.